Field of the Invention
The invention relates to an exhaust emission control system for the exhaust gas of a diesel engine, in which an injection device for a reducing agent, a mixing device and a catalyst device are disposed in an exhaust pipe.
A system of the type mentioned above is sold under the name SiNOx exhaust emission control system by the Siemens AG company, Berlin and Munich, Germany and is described in the company brochure xe2x80x9cSiNOx Stickoxidminderung fxc3xcr stationxc3xa4re Dieselmotorenxe2x80x9d (SiNOx nitrogen oxide reduction for stationary diesel engines), order number A96001-U91-A232. With such a system, the nitrogen oxide emission of a diesel engine can be drastically reduced. The mode of operation of the SiNOx exhaust emission control system is based on the SCR method (Selective Catalytic Reduction). In this case, the nitrogen oxides of the exhaust gas are mixed with a reducing agent, such as ammonia or urea, in the exhaust pipe, passed into a reactor space and converted there at the catalyst device to the environmentally harmless substances hydrogen and nitrogen. A static mixer is used for mixing the reducing agent with the exhaust gas.
From the European patent EP 0 558 452 B1 it is known to dispose in a compactly constructed exhaust emission control system three channels one behind the other, namely a pyrolysis channel, a mixing channel and a reaction channel. In the pyrolysis channel there is a two-component nozzle unit, with which aqueous urea solution is injected with compressed air into the exhaust gas stream. Here the pyrolysis takes place, i.e. the breakdown of the urea into ammonia and carbon dioxide. In the mixing channel there are three cross mixers, which provide a thorough mixing of the various substances. The thoroughly mixed exhaust gas stream is then directed into the reaction channel, where it is passed through two reduction (SCR) catalysts and then through an oxidation catalyst. After the gaseous pollutants have been eliminated, the exhaust gas stream is discharged to the outside.
When there are confined installation conditions for SiNOx diesel exhaust emission control systems, specifically in the case of land vehicles such as trucks, passenger cars, buses or locomotives, but also in the case of ocean-going vessels, such as surface vessels, ensuring an even distribution of the reducing agent in the exhaust gas proves to be a problem. This is because curves in the exhaust pipe cause an uneven distribution of the exhaust gas flow, that is to say a one-sided flow profile, and consequently an uneven distribution of the injected reducing agent. When there is a short atomizing distance due to a very small structural space, the dwell time of the reducing agent in the exhaust gas stream is not sufficient to ensure an even distribution before entry into the SCR catalyst device. As a result, the individual levels or modules in the catalyst device are subjected unevenly to the reducing agent, which results in a relatively low conversion of nitrogen oxides.
The published German patent application DE 42 03 807 A1 discloses an exhaust emission control system in which an exhaust gas inlet pipe is provided perpendicularly with respect to the main axis of an exhaust pipe. An injection device for a reducing agent, a mixing device and a catalyst device are provided in the exhaust pipe. The mixing device is provided upstream of the injection devicexe2x80x94seen in the direction of flow of the exhaust gas. The exhaust gas inlet pipe connected perpendicularly with respect to the main axis of the exhaust pipe has the effect that the exhaust gas is introduced tangentially into the exhaust pipe. The resultant swirl in the exhaust gas is utilised through the use of the mixing device for mixing the exhaust gas with the reducing agent introduced via the injection device. However, a swirl of this kind, resulting from the tangential introduction of the exhaust gas, disadvantageously leads to an uneven distribution of the exhaust gas along the radial direction.
The extent to which a thorough mixing of the exhaust gas with the reducing agent can be achieved is unsatisfactory, especially as the injection device is provided centrally in the middle of the exhaust pipe.
It is accordingly an object of the invention to provide an exhaust emission control system which overcomes the above-mentioned disadvantages of the heretofore-known systems of this general type and which, while having a short atomizing distance, ensures an even distribution of the reducing agent in the exhaust gas.
With the foregoing and other objects in view there is provided, in accordance with the invention, an exhaust emission control system for an exhaust gas of a diesel engine, including an exhaust pipe having a main axis; an exhaust gas inlet pipe aligned along the main axis of the exhaust pipe and connected to the exhaust pipe for receiving an exhaust gas having a flow direction; an injection device disposed in the exhaust pipe for injecting a reducing agent; a mixing device disposed in the exhaust pipe upstream of the injection device, as seen in the flow direction of the exhaust gas; and a catalyst device disposed in the exhaust pipe.
The object of the invention is achieved by the exhaust gas inlet pipe being aligned along the main axis of the exhaust pipe. In other words, no deflection of exhaust gas takes place when it flows through the exhaust gas inlet pipe into the exhaust pipe. The exhaust gas is introduced parallel to the exhaust pipe.
In accordance with another feature of the invention, the catalyst device may include a plurality of levels, each of which is an SCR catalyst module. Alternatively, however, the catalyst device may also include a plurality of levels of which the first level is a hydrolysis-catalyst module and every further level is an SCR catalyst module.
In order to further improve the mixing, a further mixing device can be provided between the injection device and the catalyst device.
This further mixing device is preferably provided at a point at which the exhaust pipe widens for receiving the catalyst device.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an exhaust emission control system for the exhaust gas of a diesel engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.